The invention relates to an information storage system comprising an optical record carrier having at least two information planes, and a reading device for scanning the information planes from one side of the record carrier, said device comprising a first optical system for forming a radiation spot on an information plane to be read, a second optical system for passing radiation from the record carrier to a radiation-sensitive detection system which converts the radiation into an electric detection signal, and a detection circuit electrically connected to the detection system for converting the detection signal into an information signal. The invention also relates to an optical record carrier for use in such a multiplane information storage system.
In the aim for increasing the information density in record carriers having a plurality of information planes, the distance between the information planes is to be minimized, while separate reading of the information stored in each information plane should remain possible. A first step of storing more information on a record carrier is the double-sided record carrier, i.e. a record carrier in which an information plane is present at each side. The two planes are read from different sides of the record carrier. A subsequent step is to provide two or more information planes on a record carrier, which planes can be read from one side. By securing two of these multiplane record carriers via one side, the information contents can be further increased by a factor of two.
An information storage system of the type described in the opening paragraph is known from U.S. Pat. No. 3,855,426. The reading device of the storage system focuses a radiation beam on one of the information planes of the record carrier. The information is stored in information areas, or marks in the information planes. The radiation passed by the record carrier is modulated by the marks and is received by a lens which forms an image of the area of the information plane to be currently read on a radiation detection system. The detection system converts the modulation of the incident radiation into an information signal. If the radiation beam is focused on an information plane, it will also illuminate an area on a different, higher-or lower information plane. This area or parasitic area whose illumination is unwanted should be so large and contain so many marks that the influence of the separate marks on the desired reading signal is averaged. The average influence of the marks in the parasitic area on the information signal will then be no more than a reduction of the modulation depth of the information signal generated by the interaction of the radiation beam with the marks in the plane to be read. To achieve this effect, the distance between the information planes in the known record carrier is taken to be larger than 10 .mu.m, dependent on the numerical aperture of the objective system.
Said U.S. Pat. No. 3,855,426 does not state the requirements to be satisfied by the reading device and the record carrier in order that the reading device can derive the information stored in an information plane to be read with sufficient reliability from the radiation originating from the record carrier. Knowledge of these requirements is of great importance due to the trend of increasing the information density in record carriers and the related wish of positioning the information planes closer together so that there is a greater risk of crosstalk between the information planes while the reading device becomes more critical.